Thalassemia is a common hemoglobinopathy which results in ineffective erythropoiesis and shortened red blood cell (RBC) survival. Patients with thalassemia require routine red blood cell transfusions to maintain adequate hemoglobin synthesis in the bone marrow. The timing and frequency of transfusions must be carefully planned to replace destroyed erythrocytes and suppress ineffective erythropoiesis. Excreted CO (ETCO) is an indicator of heme breakdown from both RBC breakdown and ineffective erythropoiesis and can be measured by breath analysis. This study will test the following hypotheses: 1. The elevated ETCO in thalassemia in major will primarily reflect elevated bone marrow erythropoiesis; 2. ETCO will correlate with other markers of ineffective erythropoiesis; 3. changes in ETCO between regular transfusions can be used to determine optimal time and minimal quantity of transfusion to maintain a low level of ineffective erythropoiesis.